Rapid rise in the global ocean carbon sink determined from atmospheric oxygen observations

Abstract The rate of increase of atmospheric carbon dioxide (CO2) in recent decades is only about half that expected if all anthropogenic emissions had stayed airborne. The “missing” CO2 is absorbed by the oceans and land vegetation1. Of these two sinks for CO2, the ocean sink is considered to be the better constrained2-4. Nevertheless, estimates of ocean uptake, using data products that integrate surface ocean observations5-9, suggest a sink that is larger, and increasing more rapidly since about 2000, than do global ocean models, so that they now disagree substantially1. Here we examine the recent history of the CO2 sinks using atmospheric observations of CO2 and oxygen/nitrogen ratios from globally distributed measurement locations10-12. These provide a robust and independent separation of the ocean and land contributions. This method indicates that the ocean sink increased rapidly through the years 2000-2015, to values consistent with, though generally larger than, the surface-ocean based data products, but inconsistent with models. Since 2015, our results suggest this increase has stalled, but it remains 50% higher than most models predict.